For many industrial processes it is necessary to perform distillations to separate components of a mixture. For most commercial distillations the volumes are very large, so that energy efficiencies can have significant economic impact. Finding ways to improve energy utilization is thus of great commercial interest. Another important consideration for distillation is the temperature required to separate different components. At high temperatures, the mixture may undergo unwanted reactions that limit the recovery of components. Thus, there is substantial interest in reducing the energy input and/or temperature required to separate these components.
The separation of crude oil is a major industrial process. The extraordinarily large volumes that are handled make minor improvements in efficiency and recovery of extreme economic importance. Petroleum in its unrefined state is referred to as crude oil. Commercially useful products are obtained by separation or fractionation of the crude oil by distillation into various hydrocarbon components or fractions, which fractions may be subjected to further treatment to enhance the value of the fractions. The fractions may be characterized by the average number of carbon atoms of the molecules in a fraction, the density of the fraction and the boiling range of the fraction. For classification purposes, the fractions may be designated as follows: (a) straight run gasolines, boiling up to about 390° F.; (b) middle distillates, including kerosene, heating oils, and diesel fuel, boiling in the range of about 340 to 650° F.; (c) wide cut gas oils, including waxes, lubricating oils and feed stock for catalytic cracking to gasoline boiling in the range of about 650 to 1000° F.; and (d) residual oils, including asphalts, boiling above about 1000° F.
In processing petroleum, crude oil is first desalted and dehydrated, as necessary, and may be passed through heaters where the temperature is raised. The crude oil may be raised to an elevated temperature, so that under the conditions of the process substantially all of the gasolines and middle distillates are in the vapor phase. The crude oil liquid and vapor mixture is then piped to a distillation or fractionating tower for “topping,” which represents the first step in separating the crude oil into its constituent fractions.
Up to the point of fractionation, the entire crude oil may have been heated and maintained at an elevated temperature to maintain the light fractions in the vapor phase, while maintaining the heavy fractions at a temperature that allows for a sufficiently lowered viscosity to permit the flow of the heavy fraction. There is much inefficiency in this procedure in requiring heat to allow for the separation of the light fractions from the heavy fractions and heating the entire mixture to permit this separation. In addition, the high heat required for this separation may reduce recovery of certain components due to unwanted reactions.
Shear induced phase separation (“SIPS”) has been studied in a number of systems, particularly with polymeric solutions comprising two or more components. In these studies it is found that under certain conditions of shear there is a demixing of components resulting in phases enriched for the components. By observing the composition under shear, one frequently encounters turbidity and changes in such properties as birefringence and light scattering.
To better understand the effect of shear on vapor pressure, it is necessary to appreciate that the vapor pressure of a liquid is a delicate balance between the rate of molecules escaping the surface of the liquid and the rate of molecules sticking to the surface of the liquid that strike it from the gas phase. The effect of shearing is to change the energy content of molecules at the gas-liquid interface as well as change the spatial configuration of molecules on the surface of the liquid. In some cases, it also changes the shapes of the molecules, such as inducing a transition from coiled to uncoiled. In particular, shearing can promote demixing in which the attraction between unlike molecules in a mixture is reduced. The result is for shearing to change the vapor pressure of the system by altering the rate balance described above.
When a solid or liquid is subject to a shear, a nearly instantaneous deformation occurs as if it were like a spring (Hooke's law) but this rapid deformation is often followed by a continuous one (a creep). This time-dependent response to shear is called viscoelasticity. Viscoelastic liquids can be described by different time scales for how they relax after a stress has been applied or removed.
To understand SIPS better it is necessary to appreciate the effect of viscoelasticity on phase separation. A liquid composed of two types of molecules A and B that are dissolved (mixed) can be separated (become demixed) into phases A and B under certain circumstances by the application of stress to the liquid mixture. The dynamics of the phase separation depend on the temperature, the relative concentrations of A and B, the viscoelastic properties of the mixed and demixed liquids, and the surface tension between the two phases. What is important for an understanding of this invention is that for a fixed temperature and for a fixed relative concentration, shearing can affect the solubility of A and B through their viscoelastic properties. Specifically, shearing can promote mixing or cause demixing depending on the shear rate. It is known from previous studies of polymer blends that SIPS is a common effect. Moreover, the shear induced phase separation often is sustained only by continuous shearing so that when shearing is removed or reduced, the liquid system will relax to a mixed state as a function of time unless other actions are taken, such as changing the temperature, or the relative composition of A and B, or by adding some stabilizing agent. It should be noted that the phenomenon of SIPS may occur in solutions of more than two types of molecules as well, with the complex solutions comprising crude oils being an example.
Generally, SIPS has been viewed as a neutral or even detrimental effect in industrial processes, because such processes ordinarily specify or assume the use of relatively homogeneous, well-mixed substances. It has not been understood that such separation could be intentionally affected and exploited for more efficient processing. In addition, it has not been appreciated that shearing could be used to reduce the temperature required for processing.
There is a great deal of interest in improving the processing methods used for crude oil. Because of the huge amounts of crude oil that are processed, very small improvements can have large economic consequences. It is therefore of interest to provide treatments of the crude oil and like mixtures that reduce the energy input for separating the light and heavy fractions, decrease the temperature required to separate light and heavy fractions, improve the separation into different components, increase the speed of the separation process or all of these. The subject invention addresses this issue using SIPS.